I ESTIMATED IT TO BE AROUND 24 TO 25 U.S. GALLONS...I ALSO ASSUMED THE SEMI-OVAL AREA THAT MAKES UP THE BOW TO BE TRIANGULAR FOR EASY CALCULATION PURPOSES.
17" HIGH BY 24" LONG BY 12" WIDE EXCEPT THE BOW WHICH IS ABOUT 3" WIDER AT WIDEST POINT.
17*24*12 = 4896 CUBIC INCHES
1/2 BASE * HEIGHT = 1/2 * (15-12) * 12 * 2 TRIANGLES * 17 = 612 CUBIC INCHES
4896 + 612 = 5508 CUBIC INCHES
5508 / 1728 CUBIC INCHES PER CUBIC FOOT = 3.2 CUBIC FEET
3.2 * 7.48 US GALLONS PER CUBIC FOOT = 24 GALLONS, BUT TANK IS ACTUALLY LARGER